Hello...
I'm trying to access data in a file in C. I want to read some portion
of it, modify it, then rewrite that portion back into the file.
Reading isn't a problem -- I can use fseek or lseek or whatever and read
the data I want using fread or read. No problem.
Writing is another matter. When I seek to the spot and write the
modified data, it destroys the data that comes afterwards.

That's not the way it's supposed to work. Could you post the code you're
using to open this file and write to it?